Aromatic copolyestercarbonates are well known as copolymers derived from carbonate precursors, dihydric phenols, and aromatic dicarboxylic acid or acid derivatives and are described for example in U.S. Pat. Nos. 3,030,331; 3,169,121 and 3,207,814, all of which are hereby incorporated by reference. Among the properties characterizing these polymers is a relatively high distortion temperature under load (DTUL) as well as a relatively high impact strength as measured by a notched Izod test system. The impact failure mode, however, for both the thin and thick test pieces is often brittle rather than ductile.
Aromatic polycarbonates are also well known polymers derived from carbonate precursors and dihydric phenols. Among the properties characterizing these polymers is a DTUL significantly lower than an aromatic polyestercarbonate and a high resistance to impact in thin sections. In addition, the impact failure mode for thin section test pieces is ductile rather than brittle. However, aromatic polycarbonates have a critical thickness beyond which impact strength drops quite rapidly and the impact failure mode changes from ductile to brittle. The notched Izod test system described in ASTM standard D256 utilizing 6.4 mm test samples is beyond such critical thickness for aromatic polycarbonates. Thus impact failure tests for aromatic carbonates utilizing 6.4 mm test samples show a brittle failure mode and a relatively low impact strength.